PITTSBURGH — The grove of hemlock trees around where United Flight 93 crashed on Sept. 11 is being attacked by an insect that wasn’t there 20 years ago, and some scientists say it’s an example of how climate change combines with other factors to cause environmental damage.

The problem at the Flight 93 National Memorial in southwestern Pennsylvania doesn’t involve superstorms or melting polar icecaps, but rather hemlocks battling the slow, deadly spread of a tiny creature that has only one natural predator in eastern forests — extremely cold winters.

The hemlock wooly adelgid is about the size of a match head, and for thousands of years it didn’t exist on the East Coast. Native to Asia, the insects lay their eggs on the underside of hemlock branches, and the young insects feed on the sap of the trees, often causing them to lose needles and die within five to 10 years. Left to their own devices, hemlocks can grow to over 150 feet tall, and the dense evergreen branches create a cool, shaded environment that some liken to a forest cathedral. The tree has long flourished from the Carolinas to Maine, but after the first adelgids were discovered in Virginia during the 1950s, some areas suffered heavy die-offs.

They didn’t reach southeastern Pennsylvania until roughly the early 1990s, according the U.S. Forest Service, and in 2002 they were confirmed in Somerset County, where the Flight 93 National Memorial is located.

Rick Turcotte, an entomologist with the U.S. Forest Service, can’t say that climate change is directly responsible for the adelgid infestation at the Flight 93 site. But as the climate warms, the “expectation would be that we would have larger outbreaks. As a general rule, we usually depend on winter” to be a major control for the insect, Turcotte told The Associated Press.

Michael Mann, a climate scientist at Penn State University, said that it makes sense to mention climate change in this case. While many factors influence the spread of invasive pests like the adelgid, “in this case warmer winters are indeed one of them,” Mann wrote in an email. “And we know that climate change is contributing to those warmer winters.”

Pennsylvania’s official report on the likely impacts of climate change makes the connection, too.

“For some species, hemlock for example, higher mortality rates associated with warmer winter temperatures were likely linked to greater winter survival of insects and pathogens,” the 2013 Climate Impacts Assessment notes, adding that a warming climate could allow the adelgid “to expand its range within the state.”

Climate data shows that during the same time period as adelgids have spread northwards, winters have gotten significantly less cold.

A 2012 U.S. Forest Service report notes that extreme winter minimum temperatures increased by 5.9 degrees in southeastern states between 1960 and 2004. In Pennsylvania, the state climatologist found that the average winter temperature increased by about 1.2 degrees between 1950 and 2012, and average snowfall decreased by about 5 inches a year.

The report said pests such as the adelgid “are already having major effects on forest productivity in the United States and Canada, and climate change is likely to increase their impact.”

“So we are loading the random dice of weather in such a way that favors these warm winters and the resulting impacts,” such as spread of adelgids, Mann said.

Turcotte, the Forest Service entomologist, said one thing is clear.

“I would say the stand is prettily heavily infested,” he said of the hemlocks around the Flight 93 crash site. “Trees are showing fairly significant impact,” such as reduced growth, thinning of the crown, and off-color needles.

The Forest Service is working on a treatment plan for more than 1,300 mature trees in the roughly 11-acre grove of hemlocks. They’re trying chemicals and even releasing a species of beetles that eats the adelgids.